An investigation into the structural conformations assumed by aberrant base pairs in DNA is proposed. The model system which we intend to study is comprised of a synthetic DNA polymer of known sequence interrupted by a single 2-aminopurine cytosine base mispair. 2-aminopurine, a base analogue of adenine, is a strong mutagen which can replace adenine in DNA and subsequently mispair with cytosine at relatively high frequencies in vivo and in vitro. Our specific objective is to determine if 2-aminopurine cytosine base mispairs in DNA contain either of the two bases as a disfavored imino tautomer, favored amino tautomer, or ionized base. The method proposed to address our objectives is N-15 NMR spectroscopy. The effects of nearest neighbor base stacking partners and changes in solvent conditions will also be investigated. These experiments are directed toward fundamental issues concerning the physical-chemical basis of mutagenesis at a molecular level. The experimental findings should be relevant to the medical areas of cancer, aging and metabolic disorders involving base substitutions in DNA.